Bone mineral information acquisition apparatus, bone mineral information acquisition method, and bone mineral information acquisition program

ABSTRACT

A body thickness estimation unit estimates a body thickness of a subject for each pixel of at least one of a first radiographic image or a second radiographic image which includes a primary ray component and a scattered ray component, on the basis of the at least one of the first radiographic image or the second radiographic image. A bone part pixel value acquisition unit acquires a pixel value of a bone region of the subject from the first and second radiographic images. An information acquisition unit acquires bone mineral information indicating a bone mineral content of the bone region for each pixel of the bone region on the basis of imaging conditions in a case in which the at least one of the first radiographic image or the second radiographic image has been acquired, the body thickness for each pixel, and the pixel value of the bone region.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119 toJapanese Patent Application No. 2018-100422 filed on May 25, 2018. Theabove application is hereby expressly incorporated by reference, in itsentirety, into the present application.

BACKGROUND Technical Field

The present disclosure relates to a bone mineral information acquisitionapparatus, a bone mineral information acquisition method, and a bonemineral information acquisition program that acquire bone mineralinformation using a radiographic image including a bone.

Related Art

Dual X-ray absorptiometry (DXA) has been known as a representative bonemineral quantitation method used to diagnose bone density in a bonedisease such as osteoporosis. The DXA method calculates bone mineralcontent from the pixel values of radiographic images obtained by imagingwith radiations having two types of energy levels, using the fact thatradiation which is incident on the human body and is transmitted throughthe human body is attenuated by a mass attenuation coefficient μ (cm²/g)depending on a substance (for example, bone) forming the human body, thedensity ρ (g/cm³) of the substance, and the thickness t (cm) of thesubstance.

In addition, a radiography apparatus has been known which comprises tworadiation detectors that include a plurality of pixels accumulatingcharge corresponding to emitted radiation and are provided so as to bestacked. Further, a technique has been known which measures the bonemineral content of a subject using each electric signal corresponding tothe amount of radiation emitted to each radiation detector in this typeof radiography apparatus (see JP2018-015453A).

However, in a case in which radiographic images are acquired, scatteredrays are generated due to the scattering of radiation in the subject. Inthe DXA method, the subject is irradiated with radiation such that theinfluence of scattered rays is reduced. In order to acquire bone mineralinformation using the DXA method, a dedicated apparatus for irradiatingthe subject with radiation is required as described above. Therefore, itis difficult to use the existing facilities. In addition, since the DXAmethod calculates bone mineral content for each bone, it is difficult toevaluate bone mineral information for each part of the bone.

SUMMARY

The present disclosure has been made in view of the above-mentionedproblems and an object of the present disclosure is to provide atechnique that can acquire bone mineral information using the existingfacilities.

According to an aspect of the present disclosure, there is provided abone mineral information acquisition apparatus comprising: a bodythickness estimation unit that estimates a body thickness of a subjectincluding a bone part and a soft part for each pixel of at least one ofa first radiographic image or a second radiographic image which includesa primary ray component and a scattered ray component on the basis ofthe at least one of the first radiographic image or the secondradiographic image, the first and second radiographic images beingacquired by radiations that have different energy distributions and havebeen transmitted through the subject; a bone part pixel valueacquisition unit that acquires a pixel value of a bone region of thesubject from the first and second radiographic images; and aninformation acquisition unit that acquires bone mineral informationindicating a bone mineral content of the bone region for each pixel ofthe bone region on the basis of imaging conditions in a case in whichthe at least one image has been acquired, the body thickness for eachpixel, and the pixel value of the bone region.

In the bone mineral information acquisition apparatus according to theaspect of the present disclosure, the first and second radiographicimages may be acquired by irradiating two detectors that overlap eachother with the radiation transmitted through the subject.

In the bone mineral information acquisition apparatus according to theaspect of the present disclosure, the first and second radiographicimages may be acquired by alternately irradiating one detector with theradiations that have different energy distributions and have beentransmitted through the subject.

In the bone mineral information acquisition apparatus according to theaspect of the present disclosure, the information acquisition unit mayacquire the bone mineral information by converting the pixel value ofthe bone region into a pixel value of the bone region acquired on thebasis of a reference imaging condition.

In the bone mineral information acquisition apparatus according to theaspect of the present disclosure, the reference imaging condition may bea tube voltage that is applied to a radiation source in a case in whichthe first and second radiographic images are acquired.

In the bone mineral information acquisition apparatus according to theaspect of the present disclosure, the information acquisition unit mayacquire the bone mineral information by converting the pixel value ofthe bone region on the basis of a correction coefficient correspondingto at least one of information on the reference imaging condition,information on beam hardening corresponding to the body thickness, orinformation on whether a scattered ray removal grid is present duringimaging.

The bone mineral information acquisition apparatus according to theaspect of the present disclosure may further comprise a displaycontroller that displays information related to the bone mineralinformation on a display unit.

The information related to the bone mineral information includes newinformation calculated from the bone mineral information and newinformation calculated from information other than the bone mineralinformation. In addition, the related information may be the bonemineral information.

In the bone mineral information acquisition apparatus according to theaspect of the present disclosure, the display controller may display, asthe related information, a soft part image indicating a soft region ofthe subject, a bone part image indicating the bone region of thesubject, and a composite image obtained by superimposing the bonemineral information on the first radiographic image or the secondradiographic image on the display unit, the soft part image and the bonepart image being acquired from the first and second radiographic images.

In the bone mineral information acquisition apparatus according to theaspect of the present disclosure, the display controller may displaybone strength calculated from the bone mineral information as therelated information on the display unit.

In the bone mineral information acquisition apparatus according to theaspect of the present disclosure, in a case in which the subjectincludes a plurality of bones, the display controller may display therelated information acquired for each bone on the display unit.

In the bone mineral information acquisition apparatus according to theaspect of the present disclosure, the display controller may display therelated information on a partial region in the bone region on thedisplay unit.

In the bone mineral information acquisition apparatus according to theaspect of the present disclosure, the partial region may be a cancellousbone region in the bone region.

In the bone mineral information acquisition apparatus according to theaspect of the present disclosure, in a case in which the subjectincludes a plurality of bones, the display controller may display acomparison result between the bone mineral information items of thebones as the related information on the display unit.

In the bone mineral information acquisition apparatus according to theaspect of the present disclosure, the display controller may display acomparison result between the bone mineral information items of thepartial regions in the bone region as the related information on thedisplay unit.

In the bone mineral information acquisition apparatus according to theaspect of the present disclosure, the display controller may display acomparison result between the bone mineral information and past bonemineral information acquired at different dates and times for the samesubject as the related information on the display unit.

In the bone mineral information acquisition apparatus according to theaspect of the present disclosure, in a case in which the bone region isa vertebra region, the display controller may display, as the relatedinformation, information indicating a bone fracture risk which isgenerated from spinal alignment and the bone mineral information on thedisplay unit.

The bone mineral information acquisition apparatus according to theaspect of the present disclosure may further comprise a relatedinformation generation unit that generates the related information.

According to another aspect of the present disclosure, there is provideda bone mineral information acquisition method comprising: estimating abody thickness of a subject including a bone part and a soft part foreach pixel of at least one of a first radiographic image or a secondradiographic image which includes a primary ray component and ascattered ray component, on the basis of the at least one of the firstradiographic image or the second radiographic image, the first andsecond radiographic images being acquired by radiations that havedifferent energy distributions and have been transmitted through thesubject; acquiring a pixel value of a bone region of the subject fromthe first and second radiographic images; and acquiring bone mineralinformation indicating a bone mineral content of the bone region foreach pixel of the bone region on the basis of imaging conditions in acase in which the at least one image has been acquired, the bodythickness for each pixel, and the pixel value of the bone region.

A program that causes a computer to perform the bone mineral informationacquisition method according to the aspect of the present disclosure maybe provided.

A bone mineral information acquisition apparatus according to anotheraspect of the present disclosure comprises a memory that stores commandsto be executed by a computer and a processor configured to execute thestored commands. The processor performs: a process of estimating a bodythickness of a subject including a bone part and a soft part for eachpixel of at least one of a first radiographic image or a secondradiographic image which includes a primary ray component and ascattered ray component on the basis of the at least one of the firstradiographic image or the second radiographic image, the first andsecond radiographic images being acquired by radiations that havedifferent energy distributions and have been transmitted through thesubject; a process of acquiring a pixel value of a bone region of thesubject from the first and second radiographic images; and a process ofacquiring bone mineral information indicating a bone mineral content ofthe bone region for each pixel of the bone region on the basis ofimaging conditions in a case in which the at least one image has beenacquired, the body thickness for each pixel, and the pixel value of thebone region.

According to the present disclosure, the body thickness of the subjectis estimated for each pixel of at least one of the first radiographicimage or the second radiographic image and the pixel value of the boneregion of the subject is acquired from the first and second radiographicimages. Then, bone mineral information indicating the bone mineralcontent of the bone region is acquired for each pixel of the bone regionon the basis of the imaging conditions in a case in which the at leastone of the first radiographic image or the second radiographic image hasbeen acquired, the body thickness for each pixel, and the pixel value ofthe bone region. Therefore, it is possible to acquire the bone mineralinformation without using a dedicated apparatus unlike the DXA method.In addition, since the bone mineral information is acquired for eachpixel of the bone region, it is possible to evaluate the bone mineralinformation for each part of the bone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram schematically illustrating the configurationof a radiography system to which a bone mineral information acquisitionapparatus according to an embodiment of the present disclosure isapplied.

FIG. 2 is a diagram schematically illustrating the configuration of thebone mineral information acquisition apparatus according to thisembodiment.

FIG. 3 is a block diagram schematically illustrating the configurationof a body thickness estimation unit.

FIG. 4 is a diagram illustrating a bone part image.

FIG. 5 is a diagram illustrating the relationship between the bodythickness and the contrast of a bone part and a soft part.

FIG. 6 is a diagram illustrating a look-up table for acquiring acorrection coefficient.

FIG. 7 is a diagram illustrating a soft part image.

FIG. 8 is a diagram illustrating related information displayed on adisplay unit.

FIG. 9 is a flowchart illustrating a process performed in thisembodiment.

FIG. 10 is a diagram illustrating bone mineral information displayed onthe display unit.

FIG. 11 is a diagram illustrating bone strength displayed on the displayunit.

FIG. 12 is a diagram illustrating statistical values of the bone mineralinformation displayed on the display unit.

FIG. 13 is a diagram illustrating the statistical values of the bonemineral information displayed on the display unit.

FIG. 14 is a diagram illustrating the statistical values of the bonemineral information of partial regions in the bone region displayed onthe display unit.

FIG. 15 is a diagram illustrating the statistical values of the bonemineral information of the partial regions displayed on the displayunit.

FIG. 16 is a diagram illustrating the statistical values of the bonemineral information of the partial regions displayed on the displayunit.

FIG. 17 is a diagram illustrating the statistical values of the bonemineral information of the partial regions displayed on the displayunit.

FIG. 18 is a diagram illustrating the comparison result between thestatistical values of the bones displayed on the display unit.

FIG. 19 is a diagram illustrating the comparison result between thestatistical values of the partial regions displayed on the display unit.

FIG. 20 is a diagram illustrating the comparison result between bonemineral information items displayed on the display unit.

FIG. 21 is a diagram illustrating a bone fracture risk displayed on thedisplay unit.

DETAILED DESCRIPTION

Hereinafter, an embodiment of the present disclosure will be describedwith reference to the drawings. FIG. 1 is a block diagram schematicallyillustrating the configuration of a radiography system to which a bonemineral information acquisition apparatus according to the embodiment ofthe present disclosure is applied. As illustrated in FIG. 1, theradiography system according to this embodiment captures tworadiographic images with different energy distributions and acquiresbone mineral information using the two radiographic images. Theradiography system comprises an imaging apparatus 1 and a computer 2including the bone mineral information acquisition apparatus accordingto this embodiment.

The imaging apparatus 1 performs so-called one-shot energy subtractionthat irradiates a first radiation detector 5 and a second radiationdetector 6 with X-rays which have been emitted from an X-ray source 3 asa radiation source and then transmitted through a subject H whilechanging energy. In a case in which imaging is performed, as illustratedin FIG. 1, the first radiation detector 5, an X-ray energy conversionfilter 7 which is, for example, a copper plate, and the second radiationdetector 6 are arranged in this order from the X-ray source 3 and theX-ray source 3 is driven. In addition, the first and second radiationdetectors 5 and 6 and the X-ray energy conversion filter 7 come intoclose contact with each other.

With this configuration, the first radiation detector 5 acquires a firstradiographic image G1 of the subject H obtained by low-energy X-raysincluding so-called soft rays. In addition, the second radiationdetector 6 acquires a second radiographic image G2 of the subject Hobtained by high-energy X-rays without soft rays. The first and secondradiographic images are input to the computer 2 which is the bonemineral information acquisition apparatus. In this embodiment, in a casein which an image of the subject H is captured, a scattered ray removalgrid that removes scattered ray components of the X-rays transmittedthrough the subject H is not used. Therefore, the first radiographicimage G1 and the second radiographic image G1 include primary raycomponents and scattered ray components of the X-rays transmittedthrough the subject H.

The first and second radiation detectors 5 and 6 can repeatedly performthe recording and reading of radiographic images and may be a so-calleddirect-type radiation detector that is directly irradiated withradiation and generates charge or a so-called indirect-type radiationdetector that converts radiation into visible light and then convertsthe visible light into a charge signal. In addition, it is preferable touse a so-called TFT reading method that turns on and off a thin filmtransistor (TFT) switch to read a radiographic image signal or aso-called light reading method that emits reading light to read aradiographic image signal as a radiographic image signal reading method.

A display unit 8 and an input unit 9 are connected to the computer 2.The display unit 8 is, for example, a cathode ray tube (CRT) or a liquidcrystal display and assists the input of radiographic images acquired byimaging and various types of data required for processes performed inthe computer 2. The input unit 9 is, for example, a keyboard, a mouse,or a touch panel.

A bone mineral information acquisition program according to thisembodiment is installed in the computer 2. In this embodiment, thecomputer may be a workstation or a personal computer that is directlyoperated by an operator or may be a server computer that is connected tothe workstation or the personal computer through a network. The bonemineral information acquisition program is recorded on a recordingmedium, such as a digital versatile disc (DVD) or a compact disc readonly memory (CD-ROM), is distributed, and is installed in the computerfrom the recording medium. Alternatively, the bone mineral informationacquisition program is stored in a storage device of a server computerconnected to the network or a network storage so as to be accessed fromthe outside and is downloaded and installed in the computer ifnecessary.

FIG. 2 is a diagram schematically illustrating the configuration of thebone mineral information acquisition apparatus implemented by installingthe bone mineral information acquisition program in the computer 2 inthis embodiment. As illustrated in FIG. 2, the bone mineral informationacquisition apparatus comprises a central processing unit (CPU) 21, amemory 22, and a storage 23 as the configuration of a standard computer.

The storage 23 is a storage device, such as a hard disk drive or a solidstate drive (SSD), and stores various kinds of information includingprograms for driving each unit of the imaging apparatus 1 and the bonemineral information acquisition program. In addition, the storage 23stores radiographic images acquired by imaging.

For example, the programs stored in the storage 23 are temporarilystored in the memory 22 in order to cause the CPU 21 to perform variousprocesses. The bone mineral information acquisition program defines, asprocesses performed by the CPU 21, an image acquisition process ofcausing the imaging apparatus 1 to perform imaging to acquire the firstand second radiographic images G1 and G2 which have different energydistributions and each of which includes primary ray components andscattered ray components, a body thickness estimation process ofestimating the body thickness of the subject H for each pixel of atleast one of the first radiographic image G1 or the second radiographicimage G2 on the basis of at least one of the first radiographic image G1or the second radiographic image G2, a bone part pixel value acquisitionprocess of acquiring a pixel value of a bone region of the subject Hfrom the first and second radiographic images G1 and G2, an informationacquisition process of acquiring bone mineral information indicating thebone mineral content of the bone region for each pixel of the boneregion on the basis of imaging conditions in a case in which at leastone of the first radiographic image G1 or the second radiographic imageG2 has been acquired, the body thickness for each pixel, and the pixelvalue of the bone region, a related information generation process ofgenerating information related to the bone mineral information, and adisplay control process of displaying the related information on thedisplay unit.

Then, the CPU 21 performs these processes according to the bone mineralinformation acquisition program to make the computer 2 function as animage acquisition unit 31, a body thickness estimation unit 32, a bonepart pixel value acquisition unit 33, an information acquisition unit34, a related information generation unit 35, and a display controller36. In addition, in this embodiment, the CPU 21 executes the bonemineral information acquisition program to function as each unit.However, in addition to the CPU 21, a programmable logic device (PLD)that is a processor whose circuit configuration can be changed aftermanufacture, such as a field programmable gate array (FPGA), can be usedas a general-purpose processor that executes software to function asvarious processing units. Further, the process of each unit may beperformed by a dedicated electric circuit, such as an applicationspecific integrated circuit (ASIC), which is a processor having adedicated circuit configuration designed to perform a specific process.

One processing unit may be configured one of the various processors or acombination of two or more processors of the same type or differenttypes (for example, a combination of a plurality of FPGAs and acombination of a CPU and an FPGA). In addition, a plurality ofprocessing units may be configured by one processor. A first example ofthe configuration in which a plurality of processing units areconfigured by one processor is an aspect in which one processor isconfigured by a combination of one or more CPUs and software andfunctions as a plurality of processing units. A representative exampleof this aspect is a client computer or a server computer. A secondexample of the configuration is an aspect in which a processor thatimplements the functions of the entire system including a plurality ofprocessing units using one integrated circuit (IC) chip is used. Arepresentative example of this aspect is a system-on-chip (SoC). Assuch, various processing units are configured by one or more of thevarious processors as a hardware structure.

In addition, specifically, the hardware structure of the variousprocessors is an electric circuit (circuitry) obtained by combiningcircuit elements such as semiconductor elements.

The image acquisition unit 31 drives the X-ray source 3 to irradiate thesubject H with X-rays, detects X-rays transmitted through the subject Husing the first and second radiation detectors 5 and 6, and acquires thefirst and second radiographic images G1 and G2. At that time, imagingconditions, such as an imaging dose, a tube voltage, and an SID, areset. The imaging conditions may be set by an input operation of theoperator through the input unit 9. The set imaging conditions are storedin the storage 23. In addition, the first and second radiographic imagesG1 and G2 may be acquired by a program different from the bone mineralinformation acquisition program and then stored in the storage 23. Inthis case, the image acquisition unit 31 is read from the storage 23 inorder to process the first and second radiographic images G1 and G2stored in the storage 23. In this embodiment, it is assumed that animage of the abdomen of the subject H is captured from the chest sideand the first and second radiographic images G1 and G2 of the abdomenare acquired from the chest side.

The body thickness estimation unit 32 estimates the body thickness ofthe subject H for each pixel of the first and second radiographic imagesG1 and G2 on the basis of at least one of the first radiographic imageG1 or the second radiographic image G2. Since the body thickness isestimated for each pixel of the first and second radiographic images G1and G2, the body thickness estimation unit 32 estimates a body thicknessdistribution in at least one of the first radiographic image G1 or thesecond radiographic image G2. The body thickness estimation unit 32estimates the body thickness using the first radiographic image G1acquired by the radiation detector 5 close to the subject H. However,the second radiographic image G2 may be used. In addition, weightedsubtraction may be performed between the corresponding pixels of thefirst radiographic image G1 and the second radiographic image G2 togenerate a soft part image obtained by extracting only the soft part ofthe subject H included in each of the first and second radiographicimages G1 and G2 and the body thickness of the subject H may beestimated using the soft part image. In addition, even in a case inwhich any of the images is used, a low-frequency image indicating alow-frequency component of the image may be generated and the bodythickness may be estimated using the low-frequency image.

In this embodiment, the body thickness estimation unit 32 estimates thebody thickness of the subject H using, for example, the method disclosedin JP2015-043959A. FIG. 3 is a block diagram schematically illustratingthe configuration of the body thickness estimation unit 32. Asillustrated in FIG. 3, the body thickness estimation unit 32 comprises avirtual model acquisition unit 41, an estimated image generation unit42, a correction unit 43, and a body thickness distributiondetermination unit 44.

The virtual model acquisition unit 41 acquires a virtual model K of thesubject H having an initial body thickness distribution TO(x, y). Inthis embodiment, the virtual model K of the subject H having the initialbody thickness distribution TO(x, y) is stored in the storage 23. Thevirtual model K is data virtually indicating the subject H in which thebody thickness following the initial body thickness distribution TO(x,y) is associated with each position on the xy plane.

The estimated image generation unit 42 generates a composite image of anestimated primary ray image obtained by estimating a primary ray imageobtained by capturing an image of the virtual model K and an estimatedscattered ray image obtained by estimating a scattered ray imageobtained by capturing an image of the virtual model K as an estimatedimage obtained by estimating the first radiographic image G1 of thesubject H, on the basis of the virtual model K.

The correction unit 43 corrects the initial body thickness distributionTO(x, y) of the virtual model K on the basis of the estimated image andthe first radiographic image G1 such that a difference between theestimated image and the first radiographic image G1 is reduced.

The estimated image generation unit 42 and the correction unit 43 repeatthe generation of the estimated image and the correction of the bodythickness distribution until the difference between the estimated imageand the first radiographic image G1 satisfies predetermined endconditions.

The body thickness distribution determination unit 44 determines thebody thickness distribution satisfying the end conditions to be the bodythickness distribution of the first radiographic image G1, that is, thebody thickness T(x, y) for each pixel.

The bone part pixel value acquisition unit 33 acquires the pixel valueof the bone region of the subject H from the first and secondradiographic images G1 and G2. Specifically, the bone part pixel valueacquisition unit 33 performs weighted subtraction between thecorresponding pixels of the first radiographic image G1 and the secondradiographic image G2 to generate a bone part image Gb obtained byextracting only the bone part of the subject H included in each of thefirst and second radiographic images G1 and G2, as represented by, forexample, the following Expression (1): Gb(x, y)=G1(x, y)−μ×G2(x, y) (1).In Expression (1), μ is a weighting coefficient. FIG. 4 is a diagramillustrating the bone part image Gb. In addition, the value of eachpixel in the bone region of the bone part image Gb is a bone part pixelvalue.

In this embodiment, scattered ray components may be removed from thefirst radiographic image, the second radiographic image, the soft partimage, and the bone part image by, for example, the method disclosed inJP2015-043959A. In addition, since the distances of the firstradiographic image G1 and the second radiographic image G2 from thesubject H are different from each other, the contents of scattered raysin the first radiographic image G1 and the second radiographic image G2are different from each other. Therefore, a difference between thecontents of scattered rays in the first radiographic image G1 and thesecond radiographic image G2 may be corrected.

The information acquisition unit 34 acquires bone mineral informationindicating the bone mineral content of the bone region for each pixel ofthe bone region included in the first and second radiographic images G1and G2. In this embodiment, the information acquisition unit 34 convertsthe pixel value of the bone region into a pixel value of a bone imageacquired under reference imaging conditions to acquire the bone mineralinformation.

Here, as the tube voltage applied to the X-ray source 3 becomes higherand the energy of X-rays becomes higher, the contrast of the soft partand the bone part in the radiographic image becomes lower. While X-raysare transmitted through the subject H, beam hardening in whichlow-energy components of the X-rays are absorbed by the subject H andthe energy of the X-rays increases occurs. An increase in the energy ofthe X-rays due to the beam hardening becomes larger as the bodythickness of the subject H becomes larger. FIG. 5 is a diagramillustrating the relationship between the body thickness and thecontrast of the bone part and the soft part. In addition, FIG. 5illustrates the relationship between the body thickness and the contrastof the bone part and the soft part at three tube voltages of 80 kV, 90kV, and 100 kV. As illustrated in FIG. 5, as the tube voltage becomeshigher, the contrast becomes lower. In addition, in a case in which thebody thickness is greater than a certain value, as the body thicknessbecomes larger, the contrast becomes lower. Further, as the pixel valueof the bone region in the bone part image Gb becomes larger, thecontrast of the bone part and the soft part becomes higher. Therefore,the relationship illustrated in FIG. 5 shifts to a higher contrast sideas the pixel value of the bone region in the bone part image Gb becomeslarger.

In this embodiment, a look-up table in which the reference imagingcondition is set to, for example, a tube voltage of 90 kV is prepared.The look-up table is used to acquire a correction coefficient forcorrecting a difference in contrast depending on a tube voltage at thetime of imaging and a reduction in contrast caused by the influence ofbeam hardening. In addition, the look-up table is stored in the storage23. FIG. 6 is a diagram illustrating the look-up table for acquiring thecorrection coefficient. As illustrated in FIG. 6, in a look-up tableLUT1, as the tube voltage becomes higher and the body thickness becomeslarger, the value of the correction coefficient becomes larger. In thisembodiment, the reference imaging condition is a tube voltage of 90 kV.Therefore, in a case in which the tube voltage is 90 kV and thethickness is 0, the correction coefficient is 1. In FIG. 6, the look-uptable LUT1 is two-dimensionally illustrated. However, the correctioncoefficient varies depending on the pixel value of the bone region.Therefore, in practice, the look-up table LUT1 is a three-dimensionaltable including an axis indicating the pixel value of the bone region.

The information acquisition unit 34 acquires a correction coefficientC0(x, y) for each pixel which corresponds to the imaging conditions andthe body thickness distribution (x, y) with reference to the look-uptable LUT1. Then, the information acquisition unit 34 multiplies eachpixel (x, y) of the bone region in the bone part image Gb by thecorrection coefficient C0(x, y) to acquire bone mineral informationB0(x, y) for each pixel of the bone region as represented by thefollowing Expression (2): B0(x, y)=C0(x, y)×Gb(x, y) (2). The calculatedbone mineral information B0(x, y) is acquired by capturing an image ofthe subject at a tube voltage of 90 kV which is the reference imagingcondition and indicates the pixel value of the bone part in the boneregion included in the radiographic image from which the influence ofbeam hardening has been removed.

In a case in which the image of the subject H is captured, a scatteredray removal grid for removing scattered rays incident on the first andsecond radiation detectors 5 and 6 may be used. Therefore, look-uptables corresponding to whether the scattered ray removal grid ispresent may be prepared and a look-up table for acquiring the correctioncoefficient may be selected according to whether the scattered rayremoval grid is present. In addition, look-up tables corresponding tothe types of scattered ray removal grids may be prepared and a look-uptable corresponding to the type of scattered ray removal grid used atthe time of imaging may be selected.

The related information generation unit 35 generates information relatedto the bone mineral information. In this embodiment, the relatedinformation generation unit 35 performs weighted subtraction between thecorresponding pixels of the first radiographic image G1 and the secondradiographic image G2 to generate a soft part image Gs obtained byextracting only the soft part of the subject H included in each of thefirst and second radiographic images G1 and G2. FIG. 7 is a diagramillustrating the soft part image Gs. Then, the related informationgeneration unit 35 generates a composite image Gc obtained bysuperimposing the bone mineral information B0(x, y) on the soft partimage Gs as the related information.

In this embodiment, the bone mineral information may be superimposed onthe bone part image Gb to generate the composite image Gc or the bonemineral information B0(x, y) may be superimposed on the firstradiographic image G1 or the second radiographic image G2 to generatethe composite image Gc.

The display controller 36 displays the related information on thedisplay unit 8. FIG. 8 is a diagram illustrating the related informationdisplayed on the display unit 8. As illustrated in FIG. 8, the relatedinformation is the composite image Gc.

Next, a process performed in this embodiment will be described. FIG. 9is a flowchart illustrating the process performed in this embodiment.First, the image acquisition unit 31 directs the imaging apparatus 1 tocapture images and acquires the first and second radiographic images G1and G2 having different energy distributions (Step ST1). Then, the bodythickness estimation unit 32 estimates the body thickness of the subjectH for each pixel of at least one of the first radiographic image G1 orthe second radiographic image G2 on the basis of the at least one of thefirst radiographic image G1 or the second radiographic image G2 (StepST2). In addition, the bone part pixel value acquisition unit 33acquires the pixel value of the bone region of the subject H from thefirst and second radiographic images G1 and G2 (Step ST3).

Then, the information acquisition unit 34 acquires bone mineralinformation indicating the bone mineral content of the bone region foreach pixel of the bone region on the basis of the imaging conditions ina case in which the first and second radiographic images G1 and G2 havebeen acquired, the body thickness for each pixel, and the pixel value ofthe bone region (Step ST4). In addition, the related informationgeneration unit 35 generates information related to the bone mineralinformation (Step ST5) and the display controller 36 displays therelated information on the display unit 8 (Step ST6). Then, the processends.

As such, according to this embodiment, the body thickness of the subjectH is estimated for each pixel of at least one of the first radiographicimage G1 or the second radiographic image G2 and the pixel value of thebone region of the subject H is acquired from the first and secondradiographic images G1 and G2. Then, the bone mineral informationindicating the bone mineral content of the bone region is acquired foreach pixel of the bone region on the basis of the imaging conditions ina case in which the first and second radiographic images G1 and G2 havebeen acquired, the body thickness for each pixel, and the pixel value ofthe bone region. Therefore, it is possible to acquire the bone mineralinformation without using a dedicated apparatus unlike the DXA method.In addition, since the bone mineral information is acquired for eachpixel of the bone region, it is possible to evaluate the bone mineralinformation for each part of the bone.

In the above-described embodiment, the composite image Gc obtained bysuperimposing the bone mineral information B0(x, y) on the soft partimage Gs is generated as the related information. However, the inventionis not limited thereto. The bone mineral information for each pixelacquired by the information acquisition unit 34 may be displayed as therelated information. FIG. 10 is a diagram illustrating the bone mineralinformation displayed on the display unit 8. In addition, FIG. 10illustrates only some vertebrae of the spine for simplicity ofexplanation. In this embodiment, since the bone mineral information iscalculated for each pixel, the display of the bone mineral informationmakes it possible to check the distribution of the bone mineral contentcorresponding to the value of the bone mineral information. Inparticular, in a case in which different colors are mapped and displayedaccording to the value of the bone mineral information, it is possibleto more easily check the distribution of the bone mineral content.Further, in FIG. 10, the distribution of the bone mineral content isindicated by a difference in hatching.

In addition, the related information generation unit 35 may calculatebone strength from the bone mineral information and may use thecalculated bone strength as the related information. In this case, thebone strength can be calculated on the basis of the bone mineralinformation and an index value indicating bone texture. In addition, thedensity of a trabecular structure forming the bone is used as the indexvalue indicating the texture. Therefore, the related informationgeneration unit 35 extracts a high-frequency component of the image ofthe bone region in the bone part image Gb. Any method, such as Fouriertransform, wavelet transform, or a method using a high-pass filter, canbe used as a method for extracting the high-frequency component. Then,the related information generation unit 35 calculates a variance valueof the high-frequency components for each pixel of the bone region.Here, as the density of the trabecular structure becomes lower, thecalculated variance value of the high-frequency components becomessmaller. Therefore, the related information generation unit 35calculates bone strength using the operation of the bone mineralinformation x the variance value. Here, since the bone mineralinformation and the variance value are acquired for each pixel of thebone region, the bone strength is also calculated for each pixel.

In addition, texture features by a simultaneous occurrence matrix, suchas uniformity, contrast, correlation, or entropy, may be used as theindex value indicating the texture. The simultaneous occurrence matrixis a matrix indicating the distribution of signal values of pixels in animage and represents the frequency of the signal value of a pixeladjacent to the pixel having a certain signal value as a matrix.

FIG. 11 is a diagram illustrating the bone strength displayed on thedisplay unit 8. FIG. 11 illustrates only some vertebrae of the spine forsimplicity of explanation. In this embodiment, since the bone strengthis calculated for each pixel, the display of the bone strength makes itpossible to check the distribution of the bone strength. In particular,in a case in which different colors are mapped and displayed accordingto the bone strength, it is possible to more easily check thedistribution of the bone strength. Further, in FIG. 11, the distributionof the bone strength is indicated by a difference in hatching.

In a case in which the bone strength is displayed, the bone strength maybe displayed so as to be superimposed on the soft part image Gs or maybe displayed so as to be superimposed on the bone part image Gb. Inaddition, the bone strength may be displayed so as to be superimposed onthe first radiographic image G1 or the second radiographic image G2.

In a case in which a plurality of bones are included in the first andsecond radiographic images G1 and G2, the related information generationunit 35 may generate the related information for each bone. In thiscase, a statistical value of bone mineral information for each bone maybe used as the related information. In addition, for example, the mean,median, maximum value, and minimum value of the bone mineral informationfor each bone can be used as the statistical values. FIG. 12 is adiagram illustrating the statistical value of the bone mineralinformation displayed on the display unit 8. In addition, FIG. 12illustrates only some vertebrae of the spine for simplicity ofexplanation. In this embodiment, since the statistical value of the bonemineral information is calculated for each bone, it is possible to checkthe bone mineral information for each bone. In particular, in a case inwhich different colors are mapped and displayed according to the bonemineral information, it is possible to more easily check the bonemineral information for each bone. Further, in FIG. 12, a differencebetween the statistical values of the bone mineral information isindicated by a difference in hatching.

In FIG. 12, the statistical values of the bone mineral information aremapped by different colors corresponding to the magnitudes of thestatistical values. However, as illustrated in FIG. 13, the statisticalvalue of the bone mineral information may be displayed as a numericalvalue.

In addition, the related information generation unit 35 may generate therelated information of a partial region in the bone region for one bone.In this case, the statistical value of the bone mineral information ofthe partial region can be used as the related information. Further, forexample, the mean, median, maximum value, and minimum value of the bonemineral information of the partial region can be used as the statisticalvalues. FIG. 14 is a diagram illustrating the statistical value of thebone mineral information of the partial region displayed on the displayunit 8. In FIG. 14, for simplicity of explanation, a cancellous boneregion is a partial region of the vertebra and the statistical value ofthe bone mineral information of the cancellous bone region is displayed.In this embodiment, since the statistical value of the bone mineralinformation is calculated for each bone, it is possible to check thebone mineral information of each bone. In particular, in a case in whichdifferent colors are mapped and displayed according to the value of thebone mineral information, it is possible to more easily check the bonemineral information of each bone. Further, in FIG. 14, the differencebetween the statistical values of the bone mineral information isindicated by a difference in hatching.

As such, since the related information for the cancellous bone region inthe bone region is generated, for example, the degree of activation ofosteoblasts in the cancellous bone can be checked by medication forosteoporosis. Therefore, it is possible to easily check the effect ofmedicine treatment.

In FIG. 14, the related information only for the cancellous bone regionis generated. However, as illustrated in FIG. 15, the statistical valueof the bone mineral information of a cortical bone region in addition tothe cancellous bone region may be generated and displayed as the relatedinformation.

In FIGS. 14 and 15, the statistical values of the bone mineralinformation calculated for each partial region are mapped by differentcolors corresponding to the magnitudes of the statistical values.However, as illustrated in FIG. 16, the statistical value of the bonemineral information may be displayed as a numerical value. In addition,in FIG. 16, the numerical values of the statistical values for both thecancellous bone region and the cortical bone region are displayed.However, the numerical value of the statistical value only for thecancellous bone region or only for the cortical bone region may bedisplayed.

In the above-described embodiment, the bone region is divided into thecortical bone region and the cancellous bone region. However, theinvention is not limited thereto. For example, as illustrated in FIG.17, the femur may be divided into a femoral neck region and the otherregion, the statistical values of bone mineral information for theregions may be generated as the related information and then displayed.In this case, similarly to FIG. 16, the statistical value of the bonemineral information may be displayed as a numerical value.

In a case in which a plurality of bones are included in the first andsecond radiographic images, the related information generation unit 35may generate the comparison result between the bone mineral informationitems of the bones as the related information. In this case, the relatedinformation generation unit 35 calculates the statistical value of bonemineral information for each bone and generates, as the relatedinformation, a difference value or ratio between the statistical valuesof the bone mineral information items of a certain bone as a referencebone and other bones. FIG. 18 is a diagram illustrating the comparisonresult between the statistical values for the bones displayed on thedisplay unit 8. FIG. 18 illustrates only some vertebrae of the spine forsimplicity of explanation. The numerical values of the ratios betweenthe statistical values of the bone mineral information items of theuppermost vertebra as a reference vertebra and other vertebrae among thedisplayed vertebrae are illustrated as the comparison result. As such,since the comparison result between the bone mineral information itemsof the bones is generated as the related information and is thendisplayed, it is possible to check the bone mineral content of otherbones with respect to a certain bone as the reference bone.

The related information generation unit 35 may generate, as the relatedinformation, the comparison result between the bone mineral informationitems of partial regions in the bone region for one bone. In this case,the related information generation unit 35 calculates the statisticalvalue of the bone mineral information of each partial region in the boneregion and generates, as the related information, a difference value orratio between the statistical values of the bone mineral informationitems of a certain partial region as a reference partial region andother partial regions. FIG. 19 is a diagram illustrating the comparisonresult between the statistical values of the partial regions displayedon the display unit 8. FIG. 19 illustrates only a portion of the femurfor simplicity of explanation. In addition, the numerical value of theratio between the statistical values of the bone mineral informationitems of a femoral neck region and other regions in the displayed femuris illustrated as the comparison result. As such, since the comparisonresult between the bone mineral information items of the partial regionsin the bone region is generated as the related information and is thendisplayed, it is possible to check the bone mineral content of otherparts with respect to a certain part in one bone.

In addition, the related information generation unit 35 may generate, asthe related information, the comparison result between bone mineralinformation items acquired for the same subject at different acquisitiondates and times. In this case, the related information generation unit35 calculates the statistical values of the latest bone mineralinformation and the past bone mineral information for the same subject.The statistical values may be calculated for each bone or thestatistical values of all of the bones included in the radiographicimage may be calculated. Then, the related information generation unit35 generates the comparison result between the past statistical valueand the latest statistical value as the related information. The ratioor difference value between the past statistical value and the lateststatistical value can be used as the comparison result.

FIG. 20 is a diagram illustrating the comparison result between the bonemineral information items displayed on the display unit 8. In addition,FIG. 20 illustrates, as the comparison result, the ratio between thestatistical values of the past bone mineral information and the latestbone mineral information of each vertebra. Further, FIG. 20 illustratesthe date and time when the past bone mineral information was acquiredand the date and time 50 when the latest bone mineral information wasacquired. As such, since the comparison result between the bone mineralinformation items acquired from the radiographic images acquired atdifferent dates and times for the same subject is generated as therelated information and is then displayed, it is possible to recognizethe degree of progress of the disease or the degree of medicinetreatment for the subject H. In addition, it is easy to decide atreatment plan on the basis of the degree of progress of the disease orthe degree of medicine treatment.

In a case in which the bone region is the vertebra, the relatedinformation generation unit 35 may generate, as the related information,information indicating a bone fracture risk generated from spinalalignment and the bone mineral information. For example, as illustratedin FIG. 21, in a case of a subject suffering from lateral curvature, therelated information generation unit 35 calculates a Cobb angle a as thespinal alignment and calculates the bone fracture risk on the basis ofthe Cobb angle a and the bone mineral information. Here, the Cobb angleis the angle formed between two straight lines that extend from theouter edges of the vertebrae inclined at the maximum angle above andbelow the vertebra (apical vertebra) which is the apex of the curvatureand intersect each other. In addition, the relationship between the bonefracture risk, and the Cobb angle a and the bone mineral information isdetermined by a table or a computation expression. The relatedinformation generation unit 35 calculates the bone fracture risk fromthe Cobb angle and the bone mineral information with reference to thetable or the computation expression. In FIG. 21, the calculated bonefracture risk is illustrated as a numerical value (here, 80). Inaddition, the bone fracture risk becomes higher as the numerical valuebecomes larger. As such, since the bone fracture risk is generated asthe related information and is then displayed, it is possible to guide apatient who is at high risk of bone fracture such that bone fracture isprevented.

In the above-described embodiment, the first and second radiographicimages G1 and G2 are acquired by the one-shot method. However, the firstand second radiographic images G1 and G2 may be acquired by a so-calltwo-shot method that performs imaging two times. In this case, any ofthe imaging conditions in a case in which the first radiographic imageG1 has been acquired and the imaging conditions in a case in which thesecond radiographic image G2 has been acquired may be used as theimaging conditions. In addition, in the case of the two-shot method, theposition of the subject H included in the first radiographic image G1and the second radiographic image G2 is likely to be shifted by themovement of the subject H. Therefore, it is preferable to perform theprocess according to this embodiment after the position of the subjectis aligned in the first radiographic image G1 and the secondradiographic image G2. For example, the method disclosed inJP2011-255060A can be used as the position alignment process. The methoddisclosed in JP2011-255060A generates a plurality of first band imagesand a plurality of second band images indicating structures havingdifferent frequency bands for each of the first and second radiographicimages G1 and G2, acquires the amount of deviation between thecorresponding positions in the first and second band images with thecorresponding frequency band, and aligns the positions of the firstradiographic image G1 and the second radiographic image G2 on the basisof the amount of deviation.

In the above-described embodiment, the display of various kinds ofrelated information has been described. However, a plurality ofdifferent related information items may be displayed on the display unit8 at the same time.

In the above-described embodiment, image processing is performed usingthe radiographic images acquired by the system that captures theradiographic images of the subject using the first and second radiationdetectors 5 and 6. However, the present disclosure may also be appliedto a case in which the first and second radiographic images G1 and G2are acquired using a stimulable phosphor sheet as the detector. In thiscase, the first and second radiographic images G1 and G2 may be acquiredas follows: two stimulable phosphor sheets overlap each other and areirradiated with X-rays transmitted through the subject H; theradiographic image information of the subject H is accumulated andrecorded on each of the stimulable phosphor sheets; and the radiographicimage information is photoelectrically read from each of the stimulablephosphor sheets.

What is claimed is:
 1. A bone mineral information acquisition apparatuscomprising: a body thickness estimation unit that estimates a bodythickness of a subject including a bone part and a soft part for eachpixel of at least one of a first radiographic image or a secondradiographic image which includes a primary ray component and ascattered ray component, on the basis of the at least one of the firstradiographic image or the second radiographic image, the first andsecond radiographic images being acquired by radiations that havedifferent energy distributions and have been transmitted through thesubject; a bone part pixel value acquisition unit that acquires a pixelvalue of a bone region of the subject from the first and secondradiographic images; and an information acquisition unit that acquiresbone mineral information indicating a bone mineral content of the boneregion for each pixel of the bone region on the basis of imagingconditions in a case in which the at least one image has been acquired,the body thickness for each pixel, and the pixel value of the boneregion.
 2. The bone mineral information acquisition apparatus accordingto claim 1, wherein the first and second radiographic images areacquired by irradiating two detectors that overlap each other with theradiation transmitted through the subject.
 3. The bone mineralinformation acquisition apparatus according to claim 1, wherein thefirst and second radiographic images are acquired by alternatelyirradiating one detector with the radiations that have different energydistributions and have been transmitted through the subject.
 4. The bonemineral information acquisition apparatus according to claim 1, whereinthe information acquisition unit acquires the bone mineral informationby converting the pixel value of the bone region into a pixel value ofthe bone region acquired on the basis of a reference imaging condition.5. The bone mineral information acquisition apparatus according to claim4, wherein the reference imaging condition is a tube voltage that isapplied to a radiation source in a case in which the first and secondradiographic images are acquired.
 6. The bone mineral informationacquisition apparatus according to claim 4, wherein the informationacquisition unit acquires the bone mineral information by converting thepixel value of the bone region on the basis of a correction coefficientcorresponding to at least one of information on the reference imagingcondition, information on beam hardening corresponding to the bodythickness, or information on whether a scattered ray removal grid ispresent during imaging.
 7. The bone mineral information acquisitionapparatus according to claim 1, further comprising: a display controllerthat displays information related to the bone mineral information on adisplay unit.
 8. The bone mineral information acquisition apparatusaccording to claim 7, wherein the display controller displays, as therelated information, a soft part image indicating a soft region of thesubject, a bone part image indicating the bone region of the subject,and a composite image obtained by superimposing the bone mineralinformation on the first radiographic image or the second radiographicimage on the display unit, the soft part image and the bone part imagebeing acquired from the first and second radiographic images.
 9. Thebone mineral information acquisition apparatus according to claim 7,wherein the display controller displays bone strength calculated fromthe bone mineral information as the related information on the displayunit.
 10. The bone mineral information acquisition apparatus accordingto claim 7, wherein, in a case in which the subject includes a pluralityof bones, the display controller displays the related informationacquired for each bone on the display unit.
 11. The bone mineralinformation acquisition apparatus according to claim 7, wherein thedisplay controller displays the related information on a partial regionin the bone region on the display unit.
 12. The bone mineral informationacquisition apparatus according to claim 11, wherein the partial regionis a cancellous bone region in the bone region.
 13. The bone mineralinformation acquisition apparatus according to claim 7, wherein, in acase in which the subject includes a plurality of bones, the displaycontroller displays a comparison result between the bone mineralinformation items of the bones as the related information on the displayunit.
 14. The bone mineral information acquisition apparatus accordingto claim 7, wherein the display controller displays a comparison resultbetween the bone mineral information items of the partial regions in thebone region as the related information on the display unit.
 15. The bonemineral information acquisition apparatus according to claim 7, whereinthe display controller displays a comparison result between the bonemineral information and past bone mineral information acquired atdifferent dates and times for the same subject as the relatedinformation on the display unit.
 16. The bone mineral informationacquisition apparatus according to claim 7, wherein, in a case in whichthe bone region is a vertebra region, the display controller displays,as the related information, information indicating a bone fracture riskwhich is generated from spinal alignment and the bone mineralinformation on the display unit.
 17. The bone mineral informationacquisition apparatus according to claim 7, further comprising: arelated information generation unit that generates the relatedinformation.
 18. A bone mineral information acquisition methodcomprising: estimating a body thickness of a subject including a bonepart and a soft part for each pixel of at least one of a firstradiographic image or a second radiographic image which includes aprimary ray component and a scattered ray component, on the basis of theat least one of the first radiographic image or the second radiographicimage, the first and second radiographic images being acquired byradiations that have different energy distributions and have beentransmitted through the subject; acquiring a pixel value of a boneregion of the subject from the first and second radiographic images; andacquiring bone mineral information indicating a bone mineral content ofthe bone region for each pixel of the bone region on the basis ofimaging conditions in a case in which the at least one image has beenacquired, the body thickness for each pixel, and the pixel value of thebone region.
 19. A non-transitory computer-readable storage medium thatstores a bone mineral information acquisition program that causes acomputer to perform: estimating a body thickness of a subject includinga bone part and a soft part for each pixel of at least one of a firstradiographic image or a second radiographic image which includes aprimary ray component and a scattered ray component, on the basis of theat least one of the first radiographic image or the second radiographicimage, the first and second radiographic images being acquired byradiations that have different energy distributions and have beentransmitted through the subject; acquiring a pixel value of a boneregion of the subject from the first and second radiographic images; andacquiring bone mineral information indicating a bone mineral content ofthe bone region for each pixel of the bone region on the basis ofimaging conditions in a case in which the at least one image has beenacquired, the body thickness for each pixel, and the pixel value of thebone region.